Does cheater holes make a jet faster?

[joe yap]

BTW, the difference between big fan system and a small fan system is the massflow. For a small scale fan system, the air molecules going through the ducting is lesser and all you need to do is to spin the rotor faster to achiever the same Massflow. Take an example from the fullsize RTB fantrainer, which the rotor probably spins at much lower RPM than our 30000++rpm microfan. WIth that kind of airflow velocity in out models' ducted fan system, the reynolds numders are already well taken care of.

It is that simple for the fan system. For aerodynamic wise on the airframe, it is a very much different ballgame.

[blueangel]

This subject is getting more interesting Joe.

By the way, butt dyno is a term use to describe a person whom like to judge power gain on cars ( or bikes ) after he had modified certain stuffs or component in them. Without going through the Dynometer machine to find out the actual power gain, he judges the power gain by his butt. When he sits in the car, he "felt" there is more power after the modification, which unfortunately is usually is not the case most of the time. More likely is just a Placebo Effect.

Thus I am just curious if these improvements made on a scaled down jet can ever achieve significant improvement or because one had done some work on the plane, he jolly well expect it to be better. Oh well..

From what I understand, the Red Arrow EDF plane is already unrealistic in its intake duct. It is almost twice the size in proportion to the real BAE Hawk. Even the exhaust holes are at least twice or triple of the real thing.

[joe yap]

This subject is getting more interesting Joe.

By the way, butt dyno is a term use to describe a person whom like to judge power gain on cars ( or bikes ) after he had modified certain stuffs or component in them. Without going through the Dynometer machine to find out the actual power gain, he judges the power gain by his butt. When he sits in the car, he "felt" there is more power after the modification, which unfortunately is usually is not the case most of the time. More likely is just a Placebo Effect.

Thus I am just curious if these improvements made on a scaled down jet can ever achieve significant improvement or because one had done some work on the plane, he jolly well expect it to be better. Oh well..

From what I understand, the Red Arrow EDF plane is already unrealistic in its intake duct. It is almost twice the size in proportion to the real BAE Hawk. Even the exhaust holes are at least twice or triple of the real thing.

You are comparing chicken with duck. The full size jets just need to breath enough air for combustion, as they are powered by gas turbine engines. Ducted fan system only accelerates air.

In the beginning of the thread, we are are discussing about the improvements on ducted fan system. Bringing in the gas turbine and scale effect equations, I think is a little off-topic here. Even the thread falls in the Electric ducted fan category and so , I think we shall leave the gas turbines out, in order to stay on track. If you think that there's very little you can do to improve EDF, then the discussion here is not quite relevant to you anymore.

Maybe we are still doing a little 'butt dyno' here, but there are many physics theories that can back our understandings. Most of us don't have many sophisticated equipment to provide technical figures here in this hobby, but significant physical improvement can be seen obviously fromour everyday flying. Just ask yourself, in the early years of EDFs, who come some planes can fly well even on brushed motors and nicd batteries, and yet some modern EDF can't even fly well with brushless motors and Lipo setup?

By the way, we are not trying to do major modifications on some complicated and untested theories here. What we are trying to do is to improve efficiencies by cutting down on losses, where the orginal designs deviates from the well proven theories discovered by scientist many centuries ago. We are not going space age yet, just old school stuffs.

[Raven]

lol... its about improving the wheel, not re-inventing the wheel.

[blueangel]

Nobody bring up this simple bench test ?

Static improvements at least... place plane on digital weighing scale and measure the thrust produced on the before-and-after improvement ?

I thought this is the simplest form of cheap but accurate method of seeing any improvements when you modify something. Without that proving method, I still say its butt dyno.

[joe yap]

Nobody bring up this simple bench test ?

Static improvements at least... place plane on digital weighing scale and measure the thrust produced on the before-and-after improvement ?

I thought this is the simplest form of cheap but accurate method of seeing any improvements when you modify something. Without that proving method, I still say its butt dyno.

Yep, you are right. Measuring static thrust sounds good, but in fact is totally meanless if you do not now what it means to your model. Having a lot of static thrust is just like using the 1st gear on your car. Your car will accelerate very fast, but as soon you reached the highest rpm your engine can produce, it'll stop accelarating. So with your 1:1 thrust to weight raio, so what? Your model will still be crawling on the ground, especially when you heavy model needs at least 4th ot 5th gear to take off. What most people are doing is just putting stronger motors and using more power to drag the 1st gear faster to reach the correct speed. On the other hand, I can simply sacrifice the static thrust and put a weak motor on 5th gear. All I need is a bungee to help or a strong arm to kick start, and the model will still fly.

Static thrust? An outrunner motor turning a low pitch propellor does a much better job. Who needs ducted fan here?

The test you mentioned above is equavalent to measuring the engine output torque of your car at 1st gear. This figure alone is not substantial to tell the performance of the car and using it as a benchmark will only mislead people. To make tests and measurements meaningful, we need to test the model and the fan system in both static and dynamic conditions, which covers a very vast range of airspeeds, and ideally in all amblient conditions. Apart from measuring thrusts, we also need to measure efflux velocities of the fan exhaust, the airflow pattern of the fan exhaust air, and possibly the local static pressure and temperature of the inlet and outlet of the fan duct. Otherwise, the individual figures alone can't be use to come up with a correct deduction either.

Since we do not own any of these complicated and expensive equipment to measure, we are still butt dyno. However, we still know how to do things right by the wheels invented much earlier by our pioneers.

[viper1990]

No worries, i have some pictures to help you guys visualize and some PDF documents on ducted fans.

Anyway to add on, you don't need to invest lots of money to achieve fast and high performance jet. I could scratch build one plane with the followings and it'll will beat most of the fastest jets sold in hobbyshops.

1) Suitable airfoil
2) Proper intake lip
3) Optimum intake area (not too big not to small)
4) Smooth walls inside the tube
5) Properly balanced blades

No need all the fancy expensive brushless motors , good lipos and ESC.

*just an extra note i saw somewhere in this thread, Lightening your plane's weight doesn't make it fly faster. It will accelerate faster but will not increase your top speed.

[joe yap]

*just an extra note i saw somewhere in this thread, Lightening your plane's weight doesn't make it fly faster. It will accelerate faster but will not increase your top speed.

If you model is heavy, you'll need to cruise at higher AoA, which will increase the induced drag, thus slowing down your model. If you are talking about airplane in a dive, that is another story. The wings do not need to produce lift at all, and most if not all the induce drag will be minimal, in a dive.

Of course, lightening of the model does not do anything to the ducted fan system. It is the airframe aerodynamics I'm talking about. If you want to increase your EDF models top speed, you need to look at BOTH the airframe and the fan systems, as the bigger picture. Saving the losses the fan system is one thing. The aerodynamic losses can also rob your model from useful work.

[blueangel]

So in summary, in simpler terms and possibly a task most hobbist able to DIY to improve is to have..

1. Smooth surfaces, trim all protrudings, round all intake edges.
2. Smoothen all internal walls in the intake / exhaust.
3. Have an efficient fan working with the correct size motor.

Of course, to start off with, get a properly design air frame in the first place.

Hope I can generalize.. correct me if I am wrong.

But for others, just fly. Because I know that most people don't do all these. All they care is to buy bigger motor and bigger batteries to go faster, until the plane weighs like a lump of Lipo on wings.

[joe yap]

There are actually a lot more things can be done, before spending more cash into the powerplant and fan systems. But the most important thing to do is to get the exhaust nozzle throat size correct, where most of the losses happen. Putting fairings on the back of the motor and the wires leading to it will help to recover some losses.

Extending the duct splitter to the very front of the fan will prevent the air coming in from the 2 inlet plenums from colliding.

To some extend, changing the angles stator vanes that support the ducted fan centrebody, will unswirl the exhaust airstream and thus increasing the efflux velocity.